Currently, various lasers have been available for various applications, ranging from low power semiconductor lasers for opto-electronic devices to high power solid state lasers for manufacturing, which is kin to the first laser invented by Maiman in 1967 (U.S. Pat. No. 3,353,115). In contrast to other light sources that emit incoherent light, a laser emits a coherent light beam. Theoretically, a coherent light beam can be focused in an area having a diameter of substantially the same order as the wavelength of light to produce high density of energy. The high quality of coherent light beam is typically expressed in a propagation factor M2=1.
For example, a solid state laser 100 is shown in FIG. 1, which typically comprises a solid state crystal rod 102 surrounded by a helical flashlamp 104. Light emitted by the flashlamp 104, known as pump light, is absorbed by the crystal 102 to excite the electrons in the crystal to an upper energy level. The excited electrons then return to the original energy level through an intermediate level resulting in light emission with a specific wavelength characterizing the crystal. The crystal rod 102 and the flashlamp 104 are configured in an optical cavity formed by a mirror 106 and a partial mirror 108. The resonance of the cavity and the stimulated emission in the crystal result in the emission of a coherent laser light beam 110 through the partial mirror 108.
The main merit of a solid state laser is its high power beam. The crystal rod absorbs pump light supplied by the flashlamp to its side and transforms it into a high power laser beam emitted from its face. A longer crystal rod will absorb more pump energy and thus emit a higher power laser beam.
However, since the laser beam travels through the crystal rod many times, the quality of light beam, e.g., the regularity of its wavefront, will be degraded by the inhomogeneity of the rod including the uneven thermal distribution in the rod. The quality of a laser beam is reflected in its focusability. For a perfect laser beam, the beam propagation factor M2 is 1. For example, the beam propagation factor M2 of a multi kilo-Watt solid state rod laser may be larger than 150, meaning the focusability of the beam is 150 times worse than the theoretical limit (M2=1).
A logical solution to less degradation of beam quality (i.e., smaller M2) would be shortening the crystal rod, which shortens the light path inside the rod. When the rod is getting shorter, it eventually becomes a disk. A further logical solution would be illuminating the face of the disk with the pump light instead of illuminating the side of the disk, since the face has much larger area than the side of the disk to receive the pump light.
For example, a solid state disk laser was disclosed in U.S. Pat. No. 5,553,088 (1996) to Brauch et al. The disclosed solid state disk laser 200 is shown in FIG. 2, which comprises a crystal disk 202 mounted on a heat sink 204 where a reflective layer 206 is disposed between the disk 202 and the heat sink 204. A face 208 of the disk 202 opposite to the heat sink 204 is AR (anti reflection) coated. The disk assembly including a disk 202, a heat sink 204, a reflective layer 206, and an AR coating 208 is referred to as an active mirror.
A diverging pump light 210 exiting from a light delivery device 212 such as a fiber bundle is focused on the disk 202 by a lens 214. The pump light can be provided by a laser diode or a set of laser diodes (not shown). The pump light 210 is incident obliquely on the disk 202. The pump light passes through the AR coated face 208 and the disk 202, and is reflected by the reflective layer 206 to pass the disk for the second time. The reflected pump light 210 is focused by a lens 216 on a mirror 218. The pump light is then reflected by the mirror 218 and passes the disk 202. After the pump light is reflected by the layer 206, it passes the disk 202 again, and returns to the light delivery device 212.
An optical cavity is formed by a mirror 220, a partial mirror 222, and the reflective layer 206 on the back of the disk 202 to generate a laser beam 224 oblique to the disk 202. In this way, the disk 202 is in the cavity and multiple passes of the laser beam 224 through the disk 202 are realized. The laser beam 224 exits from the partial mirror 222.
In this example, 4 passes of the pump light through the disk are demonstrated. A similar pump system that provides 4 passes of the pump light through an active mirror is also taught in US Patent Application Publication No. 2005/0152415 to Giesen et al. Although more passes of pump light through the disk are required to produce a higher power laser beam, all disclosed methods can only provide limited numbers of passes of pump light through the disk.
Another method disclosed in the same U.S. Pat. No. 5,553,088 (1996) uses four spherical mirrors and one plane mirror disposed next to the crystal disk to provide 8 passes of the pump light through the disk. For example, a pump system for generating multi-pass pump light 300 is shown in FIG. 3, which comprises four individual spherical mirrors 302, 304, 306, and 308. A diverging pump light 310 exiting from a light delivery device 312 is focused by mirror 302 on a disk assembly, which is an active mirror 314. The active mirror 314 comprises a plane crystal disk, a heat sink, a reflective layer between the disk and the heat sink, and an AR coated face of the disk opposite to the heat sink. The pump light is reflected by the active mirror 314 to mirror 306. Mirror 306 focuses the light on a plane mirror 316 disposed next to the active mirror 314. Plane mirror 316 reflects the light to mirror 308. Mirror 308 reflects the light to the active mirror 314. The active mirror 314 reflects the light to mirror 304. Mirror 304 reflects the light back to the active mirror 314, and reverses the whole light path.
The light path is as follows. Light delivery device 312→(1) spherical mirror 302→(2) active mirror 314→(3) spherical mirror 306→(4) plane mirror 316→(5) spherical mirror 308→(6) active mirror 314→(7) spherical mirror 304→(8) active mirror 314→(9) spherical mirror 308→(10) plane mirror 316→(11) spherical mirror 306→(12) active mirror 314→(13) spherical mirror 302→(14) light delivery device 312. The pump light hits the active mirror 4 times at steps (2), (6), (8), and (12). Since each hit produces two passes, 8 passes of the pump light through the active mirror are realized.
The disadvantages of this method are: (1) the number of passes of the pump light through the active mirror is limited by the number of the individual spherical mirrors (e.g., 4 individual spherical mirrors provide 8 passes), (2) the number of the individual spherical mirrors is limited by the size of the mirror, and (3) the mechanical system for supporting a plurality of individual spherical mirrors is complex and costly.
Another approach was disclosed by Stewen et al. (C. Stewen, K. Contag, M. Larionov, A. Giesen, and H. Hugel, A 1-kW cw thin disc laser, IEEE J. Selected Topics in Quant. Elect. Vol. 6, 650-657, 2000) as shown in FIG. 4(a). A diverging pump light exiting from a light delivery device 412 such as a fiber bundle is collimated by a lens 414. The collimated pump light is incident on a segment 401 of a parabolic mirror 416, which has a central hole for allowing a laser beam (not shown) generated from an active mirror 418 to get through the mirror 416.
The pump light is focused by the parabolic mirror to the active mirror 418. The active mirror 418 reflects the light to a segment 402 of the mirror 416. The mirror collimates and reflects the light to a folding mirror 420. The folding mirror translates and reflects the collimated beam to a segment 403 of the mirror 416.
Further referring to FIG. 4(b), at segment 403, the mirror reflects the light to the active mirror 418. The active mirror reflects it to a segment 404. The mirror reflects it to a second folding mirror (not shown). The second folding mirror translates and reflects it to a segment 405. The mirror reflects it to the active mirror 418. The active mirror reflects it to a segment 406. The mirror reflects it to a third folding mirror (not shown). The third folding mirror translates and reflects it to a segment 407. The mirror reflects it to the active mirror 418. The active mirror 418 reflects it to a segment 408. The mirror reflects it to a plane mirror (not shown). The plane mirror reflects it to segment 408, and the pump light reverses its light path, until it is reflected by the mirror at segment 401 toward the device 412. In this way, the pump light hits the active mirror 418 for 8 times. Since each hit produces two passes, 16 passes of the pump light through the active mirror are realized. Cross-section 422 in FIG. 4(b) is shown in FIG. 4(c). FIG. 4(a) corresponds to cross-section 424 in FIG. 4(b).
Similarly, the disadvantages of this method are: (1) the number of passes of the pump light through the active mirror is limited by the number of the folding mirrors (e.g., 3 folding mirrors provide 16 passes), (2) the number of the folding mirrors is limited by the size of the parabolic mirror and the folding mirror, and (3) the mechanical system for supporting a plurality of folding mirrors is complex and costly.
Similar methods using a lens or a mirror together with a number of discrete prisms to direct the pump beam back to an active mirror were disclosed in U.S. Pat. No. 6,778,580 (2004) to Erhard and Giesen. Accordingly, the disadvantages of these methods include: (1) the number of passes of the pump light through the active mirror is limited by the number of prisms, (2) the number of prisms is limited by the size of the prism, and (3) the mechanical system for supporting a plurality of prisms is complex and costly.
Therefore, the main disadvantage of prior-art pump systems is that only a limited number of passes of the pump light through the active mirror can be achieved with great difficulty. For example:                a system using an active mirror and a mirror (total 2 components) can provide 4 passes of the pump light through the active mirror;        a system using an active mirror, a plane mirror, and 4 spherical mirrors (total 6 components) can provide 8 passes of the pump light through the active mirror;        a system using an active mirror, a parabolic mirror, 3 folding mirrors, and a plane mirror (total 6 components) can provide 16 passes of the pump light through the active mirror.Accordingly, better methods for providing larger number of passes of the pump light through the active mirror with simplest possible optical means are desired.        